Cellulose acetate is a raw material widely used in various applications exemplified by films, fibers and the like.
In production of cellulose acetate on a commercial scale, a method using anhydrous acetic acid as an acetylating agent, acetic acid or methylene chloride as a diluent and sulfuric acid as a catalyst is commonly employed.
The above usual process of production of cellulose diacetate will hereinafter be explained.
High quality pulp having a high .alpha.-cellulose content is first beaten and then mixed with acetic acid and, thereafter, anhydrous acetic acid, acetic acid and sulfuric acid are added to achieve acetylation so that the cellulose is completely esterified. Then, a so-called aging treatment where hydrolysis is carried out until the acetyl value drops to about 55% is applied and then the cellulose diacetate thus obtained is isolated from the reaction system and finely divided and, thereafter, it is washed with water and dried to obtain cellulose diacetate flakes. Cellulose diacetate flakes adjusted to the desired viscosity and acetyl value have a very high solubility in acetone. Thus a 20 to 30 wt % solution of cellulose diacetate in acetone as a dope is passed through a filter and then extruded through a nozzle or die to evaporate the solvent, whereby it is molded into fibers or films.
In spinning or film molding of cellulose diacetate, however, acetone-insoluble gels are formed. These acetone-insoluble gels are responsible for fiber breakage and a reduction in the degree of stretching in fiber molding, and in film molding, and for the formation of fish eyes.
Thus, the removal of such acetone-insoluble gels is an important subject.
The present inventors made extensive investigations to find the mechanism by which the acetone-insoluble gels are formed, and a method of preventing the formation of acetone-insoluble gels. As a result, they have found that the acetone-insoluble gels are mainly made of cellulose acetate having a low acetyl value. Based on such findings, they investigated on a method of inhibiting the formation of cellulose acetate having a low acetyl value.
As a method of removing the component of a low acetyl value or the component of a low degree of polymerization contained in cellulose acetate, for example, an extraction method as disclosed in JP-B-48-23543 and JP-B-50-6229 (the term "JP-B" as used herein means an "examined Japanese patent publication") has been known.
The above extraction method, however, is unsuitable for practical use from an industrial point of view.
The present inventors have discovered that the formation of acetone-insoluble gels made of cellulose acetate having a low acetyl value is caused by the presence of a small amount of free acid in cellulose diacetate as obtained in the flake form and that the presence of hydrogen ion resulting from the free acid markedly accelerates the hydrolysis of cellulose acetate due to the thermal hydrolysis of the dope, thereby forming cellulose having a low acetyl value.